.calendar {
  border: 1px solid #ccc;
  padding: 10px;
  width: 300px;
  height: 250px;
  border-radius: 4px;
  margin: 10px; 
}

.header {
  display: flex;
  justify-content: space-between;
  height: 40px;
  align-items: center;
}
.days {
  display: flex;
  flex-wrap: wrap;
}
.day-header,.empty,.day {
  line-height: 30px;
  text-align: center;
  width: calc(100% / 7);
}

.day:hover {
  background-color: #eee;
  border-radius: 4px;
  cursor: pointer;
}